Molecular orbitals chemical shifts

In present work ab initio quantum-chemical calculations were performed by Gaussian 03 using density-functional theory for tetraphenylporphyrin. 6-3 lG(d, p) basis was used for all atoms, core electrons of which were simulated with LanL2 pseudopotential with corresponding 2-exponent basis for valence electrons. Theoretical valence band spectra of the molecules were obtained from calculated molecular orbitals. Chemical shift has been modeled as a change of electrostatic potential of atoms and three well-resolved nitrogen states and nine states were obtained situated very closely to each other, so they caimot be resolved ejqterimentally. 

P chemical shifts with generally more accurate ab initio, gauge-invaiiant-type, molecular orbital, chemical-shift calculations. They found that the chemical shift of a dimethyl phosphate in a g,g conformation is 3-6 ppm upheld from a phosphate in a g,t conformation. 

The electron density i/ (0)p at the nucleus primarily originates from the ability of s-electrons to penetrate the nucleus. The core-shell Is and 2s electrons make by far the major contributions. Valence orbitals of p-, d-, or/-character, in contrast, have nodes at r = 0 and cannot contribute to iA(0)p except for minor relativistic contributions of p-electrons. Nevertheless, the isomer shift is found to depend on various chemical parameters, of which the oxidation state as given by the number of valence electrons in p-, or d-, or /-orbitals of the Mossbauer atom is most important. In general, the effect is explained by the contraction of inner 5-orbitals due to shielding of the nuclear potential by the electron charge in the valence shell. In addition to this indirect effect, a direct contribution to the isomer shift arises from valence 5-orbitals due to their participation in the formation of molecular orbitals (MOs). It will be shown in Chap. 5 that the latter issue plays a decisive role. In the following section, an overview of experimental observations will be presented. 

The large downfield CSA for disilenes, and indeed the large isotropic chemical shift, is caused mainly by the great deshielding of one component of the tensor, o-,. Tossell and Lazzaretti propose that this deshielding results from a low-energy electronic transition between a o--bonding orbital in the molecular plane and the Si=Si 7r -orbital.45... 

In addition to the above prescriptions, many other quantities such as solution phase ionization potentials (IPs) [15], nuclear magnetic resonance (NMR) chemical shifts and IR absorption frequencies [16-18], charge decompositions [19], lowest unoccupied molecular orbital (LUMO) energies [20-23], IPs [24], redox potentials [25], high-performance liquid chromatography (HPLC) [26], solid-state syntheses [27], Ke values [28], isoelectrophilic windows [29], and the harmonic oscillator models of the aromaticity (HOMA) index [30], have been proposed in the literature to understand the electrophilic and nucleophilic characteristics of chemical systems. 

---